The `free radical' theory of ageing, which relates to the production of
harmful, lifetime-reducing reactive oxygen species, currently receives much
attention. This hypothesis suggests that animals live longer when they reduce
their caloric intake, because the associated drop in their metabolic rate
leads to a decreased production of reactive oxygen species. Thus, to
disentangle the relationship between metabolic rate and reactive oxygen
species, experiments involving food-restricted animals are necessary. But, as
animals also lose weight during fasting, an important question is whether
caloric restriction directly affects metabolism or, alternatively, whether any
changes in metabolic rate may simply be attributed to weight loss and
different organ composition in thinner animals.

To address this issue, Colin Selman from the Department of Aging and
Geriatric Research at the University of Florida and his colleagues in Florida
and Scotland investigated the energy expenditure and body morphology of
well-fed and food-restricted Fischer rats. The team expected caloric
restriction to result in a drop in the rodents' metabolic rate. Restricting
the food intake of 12 young and old rats by 40%, they tried to uncover how
caloric restriction, and thus a lowered metabolic rate, affects the ageing
process. Simultaneously, they measured total daily energy expenditure using
the doubly labelled water method both in ad libitum-fed and
food-restricted rats from the two age classes. To assess the impact of
morphology and lean tissue mass on the rats' total energy budgets, the team
sacrificed all the rats and performed intensive organ morphometric
analyses.

As expected, Selman and his colleagues found that food-restricted rats
expended less energy than their well-fed conspecifics. However, this was only
the case for 6-month-old juveniles. The team was surprised to find that older
food-restricted rats (aged 26 months) expended just as much energy as their
ad libitum-fed counterparts of the same age. Selman and his team
attributed this to the fact that the ad libitum food intake of
elderly rats declined from the beginning of the food intake measurements until
the end of the experiment, so that they ended up eating about the same amount
as the food-restricted rats. Thus, the energy expenditure of well-provisioned
rats was not different from that of undernourished rats. The authors generated
an interesting model to predict the energy demands of rats with different food
intakes, based on the rats' morphological variation, and compared these
predicted energy demands with their observed data. They found that
food-restricted rats spent significantly more energy than the authors
predicted from the rats' altered morphology. Selman and his colleagues
concluded that both young and old underfed rats have a significantly increased
metabolic rate when taking into account the rats' altered body condition.

At first sight, these results contradict the free radical theory of ageing,
as old rats with high levels of energy expenditure presumably have to cope
with more free radicals. Since previous studies on mice and dogs have revealed
that increased energy expenditure is associated with increased longevity, the
authors suggest that the free radical theory of ageing should be reassessed.
Can fasting rats provide us with clues to the secret of a longer life?

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